The present invention relates to metal wires with a double coating layer, in particular, steel wires for reinforcing articles made of elastomeric material, as well as to articles made of elastomeric material, especially tires for vehicles, reinforced with such wires.
It is known that to protect metal wires incorporated in elastomeric material from corrosion and to obtain good adhesion of such wires to the elastomeric material, the wires are coated with a metal alloy.
An alloy which has been in use for a long time for coating metal wires for reinforcing articles made of elastomeric material is an alloy of copper and zinc and more specifically an alloy containing approximately 60-70% copper and approximately 40-30% zinc.
As is already known, metal wires coated with a layer of alloy as referred to above show deterioration in adhesion to elastomeric material over a period of time and are not satisfactorily protected against corrosion. To overcome both disadvantages, the use of other types of alloy for coating wires has been proposed.
In particular, the use of ternary alloys of brass (Cu-Zn) with added nickel or cobalt or other metals as the third element of the alloy, for forming the coating layer of the wires has been proposed. With alloys of this type, some improvement is obtained in the problems of reducing deterioration in adhesion over a period of time in relation to elastomeric material and protection of the wires against corrosion, but such improvement is not sufficient to achieve a significant improvement of the quality level of articles made of elastomeric material reinforced in this way, especially tires.
A different solution which is satisfactory for both problems is that described in commonly assigned European patent 283,738 (U.S. Pat. No. 4,911,991).
That solution consists in coating the metal wires with two concentric, superimposed layers, both made from alloys selected from a zinc-nickel alloy and a zinc-cobalt alloy, of which the radially inner layer contains zinc in a quantity between 60% and 90% inclusive, while the radially outer layer contains nickel or cobalt in a quantity between 60% and 80% inclusive.
The metal wires in accordance with the above-mentioned European patent, while solving both the problem of achieving their protection against corrosion as well as that of adhesion to elastomeric material with acceptable deterioration over a period of time, still present certain problems for the purposes of their production on an industrial scale.
Such unsatisfactory industrial production occurs during the drawing phase which wires undergo after formation of the coating and consists of excessively frequent breaking of the said wires during wire-drawing, as well as excessive wear on drawplates.